Heretofore, system-wide power monitoring functions, which include system control as well as disturbance analysis and harmonic frequency analysis, for example, require sampling of the data from the power line referenced to time, either a local time clock or an absolute time reference, such as from a global positioning system. Digital fault recorders, for instance, require data (current and voltage values) from the power line at fixed time intervals to carry out voltage and current oscillography and harmonic analysis. Typical sampling rates are 1000 (1 k) samples per second or faster, with sampling being synchronized, as indicated above, to an internal clock or an external time source.
On the other hand, many power system protection functions, such as line distance protection algorithms, require sampling at multiples of the power system operating frequency (typically 60 Hz), to avoid phasor measurement errors. Phasors used by the protection algorithms are developed from the voltage and current values but can contain significant errors where the system frequency is other than nominal. Protective relays determine the power system operating frequency and use that frequency information to produce a sampling frequency which is a selected multiple of the operating frequency. This arrangement reduces possible errors in phasor calculations to a minimum; however, the resulting phasor measurements are not referenced to absolute time, so that synchronized phasor measurement applications to an entire power system are not possible.
As an alternative to the above-described systems, the power system operating frequency, once obtained, can be used to modify the coefficients of digital bandpass filters which are used to filter the sampled input data. Such a system provides information suitable for some protection functions, but also, since the original input data is sampled via a time-based clock (either internal or external) to provide an oscillography and harmonic analysis capability, it does not have a common i.e. absolute, time reference for the multiple protective relays and other protective devices located at different points in the power system. Accordingly, such a system is not suitable for those protective applications which require synchronized phasor measurements.
It is thus desirable to have a single, comprehensive system capable of producing synchronized phasor values, including such values capable of being used for system-wide control and disturbance analysis, as well as line protection functions within a single power system relay device.